Double-acting fluid-pressure jack



Feb. 11, 1964 R. LEDUc DOUBLE-ACTING FLUID-PRESSURE JACK 4 Sheets-Sheet 1 Filed July 6. 1960 Feb.-1l, 1964 R. LEDUC 3,120,788

V DOUBLE-ACTING FLUID-PRESSURE JACK Filed July 6,. 1960 4 sheets-snee: 2

Ff fa i Feb. 11, 1964 R. LEDuc l 3,120,788

DOUBLE-ACTING FLUID-PRESSURE JACK Filed July 6, 1960 4 sheets-sheet 3 Feb. 11, 1964 R. LEDUC DOUBLE-ACTING FLUID-PRESSURE JACK 4 Sheets-Sheet 4 Filed July 6, l960 United States Patent O Fice 3,1%),'552 DQUBLlE-itlilbli@ FLUlD-Pl-XESSURE HACK Rene Leduc, 23 Rue Henri Cloppet, Le Ves-inet, Seine-et-tlise, France Filed .luiy 6, lad, Ser. No. 41,657 Claims priority, application France .liuly 7, i959 l2 Claims. (Cl. 9l-359) This invention relates to duid-pressure jacks and their controls; also to the combination of a fluid-pressure jack with a pump-fed huid-pressure accumulator.

An object of the invention is a double-acting iluid-pressure jack, which may be of the differential piston type, in which once a movement of the piston in either direction has been started by the appropriate control action, the piston moves brislcly and continues to move until its stroke has been completed, even it the controlling member is further displaced (so long as the displacement of the controlling member is not excessive).

A more particular object of the invention is a jack as above described, the piston of which had feed and exhaust ducts in it terminating in ports which can be opened and closed by means carried by a control rod axially slidable in the piston and mechanically unconnected therewith, the arrangement being such that displacement of the control rod in either direction from a neutral range of position, whose extent may approximate to zero, and in which the feed and exhaust ports are both closed, causes the piston to move in the opposite direction.

To ensure that the stroke of the jack piston, when once started, is completed without fail (provided the control rod is not grossly mishandled) a further object of the invention is a jack as above described, in which the means carried by the control rod for bringing about opening and closing of the feed and exhaust ports each have an axial extension at least equal to the full stroke of the piston; and preferably, but not necessarily, so constructed that the travel of the control rod, relatively to the piston, in either direction from the neutral range, is at least equal to full stroke ot the piston.

With an arrangement as above described, movement of the piston (in either direction) once started by a displacement, from the neutral position range, of the control rod in the opposite direction, proceeds at an accelerated rate until the piston-stroke is completed, unless the control rod is displaced, relatively to the piston, in the reverse direction, i.e. the same direction as the movement of the piston, through a distance sufficient to bring it back into the neutral position range, which, it is to be remembered, is a range ot position relative to the piston; so that, in eilect, to stop the piston before it has completed its stroke, the control rod must be displaced, in the same direction as the movement of the piston, quickly enough to catch the piston up.

When a iluid-presssure jack as above described, more particularly a hydraulic jack, is combined with a pumpied hydraulic pressure accumulator, a still further object of the invention is an arrangement, in which the accumulator pressure is applied to a movable member, in opposition to the eliort of resilient means, such as a spring or a compressed gas, which member is operatively connected to the control rod of the jack, and which includes a relief valve, which when open by-passes the pump-delivery direct to exhaust, such relief valve being so controlled by the jack piston that an increase of accumulator pressure displaces the control rod in the direction to cause the jack piston to execute its stroke (in the opposite direction) and thereby open the relief valve, and conversely, in such a way that if the accumulator pressure exceeds a predetermined, upper, critical limit, the pressure is relieved, and if it falls below another predetermined lower critical ZS Patented Feb. l1, 1964 limit, the accumulator is replenished by the pump. The utility of this feature is hereinafter explained.

Such a combined arrangement may also include a nonreturn valve in a connection between the pump and the liquid chamber' or' the accumulator, operative to prevent back-flow from the accumulator towards the pump.

It may also include means, carried by the control rod of the jack, operative to open both the feed and exhaust ports of the ducts within the jack piston, should the accumulator pressure exceed a predetermined, high, critical limit, as might occur as a result oi thermal expansion of the hydraulic liquid.

These various objects of the invention will be more fully explained hereinafter, in the non-limitative description, with reference to the accompanying drawings, of certain embodiments of the invention.

The invention has numerous practical applications.

(a) For operating a circuit-breaker 'm an electrical transmission network, especially a high tension network: in this case, the circuit must be broken quickly and with complete certainty when a prescribed limiting tension (voltage) is exceeded, and it is essential that a circuitbreaking action, once started, shall not be interrupted, even if the tension immediately falls belov,I the critical limit: a hydraulic (or huid-pressure) control system according to this invention ensures this; and moreover ensures an automatic and positive reconnection of the circuit when the tension has fallen below a presecribed, lower, critical limit;

(b) For operating gates oi hydraulic and the like power systems, especially in hydro-electric installations. ln such cases, a common requirement is that the gates should open completely when a certain maximum upstream water-level is reached, and close completely when this water-level is lowered below a certain minimum: a heat-controlled system according to this invention will satisfy these requirements.

Modern hydraulic circuits are usually of the closedcenter type. Many of them comprise a feed-pump, a reservoir or sump from which the pump draws the liquid (and which receives the liquid exhausted from the service circuit) and an accumulator serving as a reserve of energy. To avoid useless consumption of energy, and incidentally thereby to increase the life of the pump, it is desirable to return the pump delivery directly to the reservoir with the least possible loss of head, when the accumulator is iull or has attained the design pressure. lt is likewise desirable to rte-establish the normal circuit, i.e. to malte the pump deliver to the accumulator, when the accumulator volume has decreased to a prescribed extent or its pressure has fa len to a prescribed, lower, critical limit.

These requirements are met by the relief valve feature of the invention, previously mentioned.

The objects and features ot the invention already mentioned and how they may be put into practice, will be better understood from consideration of the following description, having reference to the accompanying drawings, of several embodiments of the invention, given by way of example only and without implied limitation of the scope of the invention, as defined in the hereto appended claims.

ln the drawings:

FEGURES l to 3 are schematic views, in axial section of three elementary embodiments of the invention;

FlGURES 4 to 7 are axial sections of a practical embodiment of the jack and its control or the invention, intended for combination with a pump-fed pressure accumulator and including a relief valve, respectively showing four different settings of the assembly;

FiGUR 8 to .ll are similar views of an alternative embodiment, including the accumulator,y which in this snaar/ss 3 instance is unitary with the jack and its control means; and

FlGURES l2 to l5 are similarviews of another alternative embodiment, similar principle to that of FGURES 8 to lll, but incorporating means for simultaneously opening the feed and exhaust ports of the jack piston, when the accumulator pressure exceeds a prescribed upper critical limit.

'In what follows left and right are to be read as seen in the ligure unless this is repugnant to the context.

The embodiment of FGURE 1 is a double-acting jack with differential piston. lt comprises a cylinder 3, a piston l whose right hand face is of smaller area than the left hand face, and a control rod lll axially displaceable with respect to the piston. The right hand chamber of the cylinder is permanently connected to the external feed line 2. In the piston are formed a feed duct 4 and an exhaust duct 6 communicating with an external exhaust connection at 7. The feed and exhaust ducts terminate respectively in feed and exhaust ports formed in an axial bore of the piston in which slide enlargements or lands d, 9 formed on a control rod in the piston is also formed a duct 5 leading from the left hand face of the piston to a port situated half-way between the feed and exhaust ports.

Each of the lands 3, 9 has an axial extension greater than the full stroke -of the piston in the cylinder, and the distance separating these lands is slightly less than that separating the feed and exhaust ports, so that when the control rod is in neutral position (or range of position, whose extent in the illustrated example is almost nil) with respect to the piston, both the feed and exhaust ports are covered by the lands 8, 91 as shown in the figure. The possible `travel of the control rod, relatively to the piston, in either direction from this neutral position, is greater than the full stroke of the piston.

lf the control rod is displaced to the left from the position shown, the feed port (of duct d) Will be uncovered by land t? and the exhaust port (of duct d) will remain covered by land 9. Fluid under the feed pressure will thus be delivered, via ducts 4 and 5, to the left hand chamber of the cylinder and since the area of the left hand face of the piston is greater than that of its right hand face the piston will be driven to the right and this movement will accelerate until the feed port of duct 4- is fully uncovered and will continue until the piston has completed its stroke. To reverse the stroke `of the piston the control rod must be moved to the right relatively to the piston until it has passed through the neutral position shown in the gure and the land 8 has covered the feed port (4) and uncovered the exhaust port (d) thus disconnecting the left hand cylinder chamber from the external feed :2 and connecting to the external exhaust connection 7.

Since the left hand end of the bore of piston l is blind, it is connected to exhaust by an extension of the exhaust duct 6 to prevent hydraulic lock.

ln FGURE 2, the lands d, 9 of FlGURE l are replaced by cam-faced enlargements of the control rod, which as before are each longer than the full stroke of the piston and these enlargements are operative to unseat balls 8a, 9a, which, when seated, close the feed and exhaust ducts 4, 6, respectively. Otherwise the embodiment of FIGURE 2 operates `in the saine way as that of FIG- URE l.

`ln the embodiment `of FIGURE 3, the two faces of piston l are of equal area and the feed and exhaust ports are duplicated, those communicating with the left hand chamber of the cylinder 1* being controlled by ball-valves 8a, Sb, respectively, and those communicating with the right hand cylinder chamber by ball-valves Sla, 9b, respectively. These valves are arranged in the piston l and the feed valves 3a, 9u communicate, via an annular clearance d3 around the piston, with the external feed Z, while the exhaust valves El?, 9b communicate, Via a longitudinal duct in the piston with the external exhaust connection 7.

Balls Su, 3b are respectively unseatable by cam-faced enlargements illu, `llb of the control rod lil and balls 9a, 2lb by similar enlargements lila, 12b, each of which is longer than the full stroke of the piston. 'n the hgure the control rod .is in the neutral position, all the ballvalves being closed. It will be evident yfrom inspection of the ligure that leftvvard movement of the control rod will unseat balls da and '9b and connect the left hand cylinder chamber to the external feed and the right hand cylinder chamber to exhaust, causing the piston to move to the right and complete its stroke, rightvvard movement of the control rod producing the contrary result.

FEGURES 4 to 7 illustrate the application of a aci device as illustrated in FGURE Z to the automatic control of the charging of a pump-fed hydraulic pressure accumulator. The pump (not shown) feeds the accumulator (not shown) through a non-return valve (not shown). The device comprises a body Ztl, in which is formed the jack cylinder 26, in which slides the piston Ztl, the latter having a central bore in which slides the control rod 25" having cam-faced enlargements 33, 35 for unscating balls 34, 3d controlling the feed and exhaust connections of the left hand cylinder chamber as in PEG- URE 2. Elements 2a, 23, 2,7, 33, 35, 34, 3e of FGURES 4 to 7 correspond respectively to elements 3, l, lil, l2, d 9u and da of FGURE 2.

The body Ztl also encloses a chamber 2l communicating by a duct 22. with the right hand chamber of cylinder 2iand with an external feed line connected to the iquid chamber of the accumulator. `Chamber 2l is of variable `Volume being closed by a sliding partition 23 resiliently loaded by a spring =24 in opposition to the accumulator pressure in chamber 2l. Partition 23 is rigidly connected to the control rod Z7. The body 2li* also has internal ducts 3), 3l, respectively connected to the pump delivery and to the accumulator as indicated by arrows Fl, E2 and a by-pass duct '32 connected to exhaust into which the exhaust duct 37 (corresponding to duct 6 of HGURE l) of the jack piston 2S delivers.

The ducts Sie and 3l communicate with the oy-pass duct 32 `through an opening on which a springdoaded ball is seatable `(see FJ^URES 4, 7). Ball is unseatable by an extension of the jack piston 23 when the latter completes its leftward stroke `(see FGURES y5, 6). in this arransement, Ithe range of neutral travel the control rod 27 is of quite appreciable extent.

This arrangement operates as follows: With the jack piston 28 in the position of FIGURE 4, the ball 29 is seated and the pump charges the accumulator through ducts 3d, 3l (arrows El, F2). As the accumulator pressure rises the partition 23 moves to the right compress ing spring 25. ln FIGURE 4 the pressure has nearly reached the prescribed maximum limit and the control rod has nearly reached one limit of its neutral range. ln FGURE 5, the accumulator pressure has reached the prescribed maximum limit and the further rightward movement of partition and control rod 27 has unseated the ball 34, thus allowing the left hand cylinder chamber to be exhausted and causing the piston 28 to complete its stroke to the left and unseat ball 29, thus by-passing the pump-delivery to exhaust via dust 32.

When the accumulator pressure, no longer kept up by the pump falls (as a result of use of the services supplied by the accumulator) the partition Z3 and rod 27 move to the left and when the pressure has nearly fallen to a lower prescribed critical limit the control rod has nearly reached the other limit of its neutral range as shown in FIGURE 6. When this lower limit of pressure is attained the leftward movement of the partition 23 and rod 27 has unseated the ball 36, the ball 34 being already seated, so that the left hand cylinder chamber is connected to the duct 22, which is at accumulator pressure, and disconnected from exhaust, so that the piston Z8 is moved to the right through its full stroke and allows the ball 229 to be re-seated by its spring as shown annoys@ in FIGURE 7. The pump delivery is now no longer by-passed to exhaust but feeds the accumulator once more and builds up pressure therein until the situation shown in FIGURE 4 is reached, thus initiating another cycle of movements.

The embodiment of FIGURES 8 to 11 incorporates the accumulator itself, but is otherwise similar to that of FGURES 4 to 7. FIGURES il to 11 correspond respectively to FIGURES 4 to 7 and show corresponding positions of the parts. The elements numbered 2.0, 22, 217, 2S, 29, 30, 32, 37 in FIGURES 4 to 7 are designated in FIGURES 8 to l1 by 4t?, 5l, 45, 46, Sti, 47, t?, S2, respectively.

In FIGURES 8 to l1, the variable volume liquid chamber 41 of the accumulator is within the body d@ and takes the place of the chamber 2l of FIGURES 4 to 7. It is separated by the movable partition 42 (corresponding to partition 23 of FIGURES 4 to 7) from a chamber 43 containing compressed gav which constitutes the resilient means opposing the liquid pressure in chamber 4l. The latter is fed from the pump (not shown) by a continuation of duct 47 containing a spring-loaded nonreturn valve 4d, which prevents bacll-ilow from the accumulator towards the pump. The external connection of duct 51, subjected to accumulator pressure, leads to the service line of the accumulator. In this case the movable partition 42 is connected to the control rod 45 by a coupling 44, 45.

The arrangement operates in much the same way as that of FlGURES 4 to 7, which has already been described, the only difference being that the quantity determining the action of the jack-piston 46 is the volume of the chamber 41, rather than the pressure therein. This is because the spring 24 of FIGURES 4 to 7 exerts a substantially constant effort, the travel of partition 23 of FlGURES 4 to 7 being relatively small, Whereas the ctr'ort exerted by the gas pressure in chamber 43 of Flf- URES 8 to l1 is substantially inversely proportional to the volume of chamber 43.

lille embodiment of FlGURES l2 to l5 is similar in lprinciple to that of FIGURES 4 to 7, reference characters common to both sets of ligures designating corresponding elements.

In FIGURES 12 to l5, the scalable element of the bypass control valve 29 is constituted by the right hand end of the jack-piston 2S which is loaded by a spring 62 in a closed chamber 63, which spring serves only to overcome frictional effects, when the accumulator pressure is nil or very low. The movable member deiining the variable volume of the chamber 2l is the left hand end o3 of the control rod 27; and since the fluid pressure counteracted by the spring 24 is transmitted through the control rod 27, the latter is not rigidly connected to the abutment of spring 24, as in FIGURES 4 to 7, but has a push-only connection therewith.

The embodiment of FIGURES l2 to 15 also includes a non-return valve el, corresponding to the non-return valve 48 of FIGURES 8 to l1, operative between the pump-delivery duct 3l) (arrow F1) and the accumulator feed duct 3l (arrow F2) which has a branch Sla leading to the service line (arrow F4) of the accumulator. The exhaust duct T37 is constituted by the annular clearance between the control rod 27 and the extension 29 of the jack-piston Ztl constituting the by-pass valve for the pumpdelivery; and this clearance communicates, via the chamber Z5 containing spring Z4 with exhaust outlet duct 32 (arrow F3).

In the embodiment of FIGURES l2 to l5, the neutral range of travel of the control rod, relatively to the jackpiston, is of negligible extent (see FIGURE 13, in which the position of the several elements corresponds to that of FIGURE 4). The position shown in FIGURE l2 corersponds to that of FIGURE 7, it being appreciated that in the embodiment of FIGURES l2 to l5 tne smaller area face of the jack-piston is the left hand face, not the right hand face as in FIGURES 4 to 7. The position shown in FlGURE 14 corresponds to that shown in FIG- URE 5.

The embodiment of FIGURES 12 to l5 also includes an additional safety device for relieving excess accumulator pressure, due for instance to thermal expansion of the hydraulic liquid.

Normally, after the ball A, corresponding to ball 36 of FIGURES 4 to 7, has been unseated to apply the accumulator pressure to the larger area (right hand) face of the jack-piston 2S as shown in FIGURE 14, the control rod 27 will be displaced leftwards as the accumulator pressure falls, ultimately causing ball A to be reseated and the ball B, corresponding to ball 34 of FIG- URES 4 to 7, to be unseated, thus bringing about an abrupt return to the position of FIGURE l2.

However, if the accumulator pressure increases, eg. due to thermal expansion, this increased pressure acting on the end 6d of the control rod, Will cause a continued displacement of the control rod 27 to the right, as shown in FIGURE 15.

The additional safety device consists essentially in providing on the control rod 27, a further cam-faced enlargement 64 facing the opposite way to the cam-face of the enlargement which normally unseats ball B, so positioned that, when the control rod is displaced rightwards from the position shown in FIGURE l5, both the balls A and B are unseated, thus connecting 'the accumulator directly with the exhaust outlet.

Such an excess of pressure fed to the system could also occur if the jack-piston 28 were to jam in the position shown in FIGURE 13.

It will be understood that the invention is not limited to the embodiments 'hereinbefore particularly described reference to the `accompanying drawings, but includes all such modifications, variations and adaptations as are within the competence of those skilled in the art and are within the scope and spirit of the invention as defined in the hereto appended claims.

Having thus described the invention, whim is claimed as new and desired to be secured by Letters Patent is:

1. A `double-acting fluid-pressure jack including a cylinder, a piston movable in said cylinder `and having formed therein a feed duct and an exhaust duct, respectively communicating with a `feed port and an exhaust port, a control rod coaxially siidable in said piston and mechanically unconnected therewith, and means carried by said rod for bringing about opening and closure of said feed and exhaust ports respectively, each of said means having a longitudinal extension superior to its own free stroke inside the said piston in either `direction from a neutral range position, said feed and exhaust ports being so positioned that movement of said control rod in either direction from a neutral range of position, the extension of which may approximate to zero, in which both the feed and exhaust ports are closed, brings about a movement of the piston in the opposite direction.

2. A fluid-pressure jack `as defined in claim 1, in which the piston is of the dilferential type, having opposed faces of unequal area, that having 'the smaller area being constantly subjected to the feed pressure.

3. A fluid-pressure jack .as defined in claim l, in which the travel of the control rod relatively to the jack pistou, in either direction from the said range of neutral position, is at least equal to 4the total stroke of said piston.

4. A fluid-pressure jack as delincd in claim rl, including positively seatable valve members, such as balls, for closing said feed and exhaust ports, the means carried by said control rod for bringing about opening land closure of said ports comprising cam means adapted to unseat said valve members.

5. A hydraulic system comprising, in combination, cylinder means having a pair of end walls; piston means elidable in said cylinder means lbetween a pair of end positions and having a pair of opposed yfaces defining a pair 6 of cylinder chambers in said cylinder means between said opposed faces and said end Walls, said piston means being hollow and said piston means and cylinder means being formed with passage means for placing said chambers in communication with a source of liquid under pressure; and control means slidably received in said hollow piston means, said control means, piston means, and passage means icoopenating tto provide, in response to movement of said control means in one direction relative to said piston means, a pressure differential between said chan bers which moves said piston means in an opposite direction in said cylinder means from one end position to the other end position.

6. A hydraulic system as recited in claim and wherein one portion of said passage means at least part of which is formed in said cylinder means provides communication between the source of liquid under pressure and one of said cylinder chambers and another portion of said passage means which is `formed in said piston means directs the liquid under pressure to the other of said chambers, said passage means havinfy a third portion providing communication at least between said other chamber and a discharge of loev pressure, said passage means including a pair of ports respectively forming parts of said other and third portions of said passage means and said control means cooper-ating with said ports.

7. A hydnaulic system comprising, in combination, cylinder means having a pair of opposed end walls; hollow piston means slidable in said cylinder means between a pair of end positions and having a pair of opposed faces which define 'a pair of cylinder chambers with said end Walls, said cylinder means and piston means being formed with `a passage means providing communication of a source of liquid under pressure with both of said charnbers and communication of at least one of said chambers with a low pressure discharge, said passage means including in said piston means at least `a pair of ports through one of which Ithe liquid under pressure iows to said one chamber and through the other of which the liquid lows from said `one chamber to said low pressure discharge; and an elongated control rod received in and axially movlable with respect to said pis-ton means, said control rod carrying a pair of elongated means for covering and uncovering said ports, said pair of elongated means coopcrafting respectively with said ports for providing, by cooperation of said cylinder means and piston means and in response to movement of said control rod `tl'irough a slight distance from a neutral position with respect to said piston means in one direction relative thereto, a pressure advancing said piston means in said cylinder means along the entire stroke of said piston means from one to the other of its end positions, and, in response to movement of said control rod from said neutral position through a slight distance in said opposite direction relative to said piston means, a pressure returning said piston means in said cylinder means through its entire stro-lie in said one direciton.

S. A hydraulic system as recited in claim 7 and wherein said pair of elongated means each have a length greater than the stroke of said piston means.

9. A hydraulic system as recited in claim S and wherein said pair of elongated means are in the form of lands.

l0. A hydraulic system as recited in claim 8 and Wherein sa pair of ball-valve means are respectively carried by said piston means at said ports and wherein said pair ot elongated means of said control rod have irusto-conical portions respectively aligned with said pair of ball-valve means in the neutral position of said control rod and a pair of cylindrical portions respectively extending from the larger ends of said fnustoconical portions, the frustoconical portions acting on said ball-valve vmeans to open one or the other of said valve means depending upon the direction of movement of said control rod from its neutral position and the cylindrical portions oi said control rod .maintaining the opened valve means in its open position CFI during the movement of said piston means in a direction opposite to that in which the control rod is moved.

l1. A hydraulic system, comprising in combination, cylinder means having la pair of opposed end Walls :and piston means slidable in said cylinder means and having a pair of opposed Kfaces of different `areas respectively defining small-area and large-area cylinder chambers with said end Walls, said cylinder means being formed with a passage providing communication between said small-area chamber and a source of liquid under pressure and said piston means being hollow and being formed with a lirst passage leading from said small-area chamber to said largearea chamber and with `a second passage leading from said large-area chamber to a low pressure discharge, said first and second passages respectively including a pair of ports; an elongated control rod received in and axially movable Vwith respect to said hollow piston means, said control rod carrying a pair of elongated means cooperating with said ports for opening the port of said first passage and closing the pont of said ceco-nd passage in response to movement of said rod in one direction relative to said piston means, thus directing the liquid under pressure to the piston tace of larger area for advancing said piston means in an opposite direction from one end position to another end po` n in said cylinder means, iand for closing the port of said first pressage and opening the port of said second passage for releasing liquid from said large-area charnber to said low pressure discharge so that the liquid under pressure will advance said piston means in said one direction in response to movement of said control rod in said opposite direction.

12. A hydraulic system comprising, in combination, elongated `cylinder means having `a pair of opposed end walls; elongated hollow piston moans slidable in said cy-linder means and having a pair of opposed faces of equal areas defining ia pair of cylinder chambers with said end walls, said piston means being formed with an elongated cutout spaced from and located between its opposed faces yand said cylinder means being formed with a passage communicating with said cutout and directing liquid under pressure thereto, said piston means being formed with passage means leading from said cutout to `both of said chambers and from both of said chambers to a low pressure discharge and said passage means including tour ports; 4and elongated control means received in said hollow piston means and cooperating with said ports for opening one pair of ports and closing the other pair of por-ts in response to movement of said control means in one direction relative to said piston means for providing between said cylinder chambers a pressure differential advancing said piston means in an opposite direction, said control means cooperating with said ports for maintaining said pressure dii'erential until said piston means moves through an entire stroke from one end position to another end position in said cylinder means, and said control means cooperating with said ports for closing said one of ponts .and opening said other pair oi ports in response to movement ci said control means in said opposite direction relative to said piston means for providing between said cylinder chambers a pressure diilerential advancing said piston means in said one direction, said control means cooperating with said ports for maintaining said latter pressure diierential uri-til said piston means moves through its entire stroke from said other end position back to said one end position in said cylinder means.

References @ited in the tile of this patent UNITED STATES PATENTS 

1. A DOUBLE-ACTING FLUID-PRESSURE JACK INCLUDING A CYLINDER, A PISTON MOVABLE IN SAID CYLINDER AND HAVING FORMED THEREIN A FEED DUCT AND AN EXHAUST DUCT, RESPECTIVELY COMMUNICATING WITH A FEED PORT AND AN EXHAUST PORT, A CONTROL ROD COAXIALLY SLIDABLE IN SAID PISTON AND MECHANICALLY UNCONNECTED THEREWITH, AND MEANS CARRIED BY SAID ROD FOR BRINGING ABOUT OPENING AND CLOSURE OF SAID FEED AND EXHAUST PORTS RESPECTIVELY, EACH OF SAID MEANS HAVING A LONGITUDINAL EXTENSION SUPERIOR TO ITS OWN FREE STROKE INSIDE THE SAID PISTON IN EITHER DIRECTION FROM A NEUTRAL RANGE POSITION, SAID FEED AND EXHAUST PORTS BEING SO POSITIONED THAT MOVEMENT OF SAID CONTROL ROD IN EITHER DIRECTION FROM A NEUTRAL RANGE OF POSITION, THE EXTENSION OF WHICH MAY APPROXIMATE TO ZERO, IN WHICH BOTH THE FEED AND EXHAUST PORTS ARE CLOSED, BRINGS ABOUT A MOVEMENT OF THE PISTON IN THE OPPOSITE DIRECTION. 